Automatic focus control means for cathode-ray tubes



Sept. 11, 1951 AUTOMATIC FOCUS CONTROL MEANS FOR CATHODE-RAY TUBESAmply'z er Defeciar 6 1 4171 l fier ojclllaioy Modulaz or' Amplifier R.0. HoLBRooK 7 2,567,377

Filed Dec. 6, 1949 IN VEN TOR.

Ball Z 772 izolbrook Patented Sept. 11, 1951 AUTOMATIC FOCUS CONTROLMEANS FOR CATHODE-RAY TUBES Ralph 0. Holbrook, Takoma Park, Md.,assignor to Stromberg-Carlson Company, a corporation of New YorkApplication December 6, 1949, Serial No. 131,362

This invention relates to automatic focus control for cathode ray tubes,more particularly to means for automatically controlling the focusing ofpicture tubes for television receivers over a 'wide range of brightnesssettings, and the invention has for an object the provision of improvedautomatic control apparatus of the above character.

In television receivers heretofore known, the brightness and the focuscontrol means for picture tubes or cathode ray'tubes of the magneticfocus type have included adjustable means so connected either in seriesor parallel relation that the focus control means have been whollyindependent of the brightness control, with the result that changes inthe brightness control means after proper focusing of the picture tubehave resulted in defocusing, therefore requiring readjustment of thefocusing means for each change in the brightness adjustment beyond arelatively small range. Since adjustment of the brightness control isrequired by the different visual conditlons encountered in day and nightusage of television apparatus'and by the different visualcharacteristics or capacities of the users, the necessity for adjustmentof the focus for each major variation of the brightness or intensity hasoccasioned difficulties, and it is accordingly a further object of thepresent invention to provide reliable and economical meansforautomatically maintaining the focus of the cathode ray or picturetube over a wide range of variation of brightness while at the same timepermitting proper initial adjustment of the focus at any setting of thebrightness control. I

In carrying out the invention in one form, a magnetic focus type cathoderay' tube having independent focus and brightness control means is 4Claims. ,(Cl. 315-31) shown for purposes of illustration as embodiedprovided with additional control means effectively operable by thebrightness control means for maintaining the tube in focus over a widerange of adjustment of the brightness control means. More particularly,the focus control means includes a pair of adjustable potentiometers, soarranged that the focus control circuit includes an adjustable portionof. one of the potentiometers and. a constant portion of the other, andan adjustable portion of the other potentiometer is connected to controlthe brightness by applyin a variable bias-voltage to the tube. Automaticcontrol' of the focus is obtained, in accordance with the presentinvention, by connecting additional impedance means in parallel circuitrelation with the adjustable portion of the brightness controlpotentiometer, which impedance means is effective to vary the currentflow in the focus controlling-circuit upon adjustment of the brightnesscontrolling potentiometer, thereby to maintain the tube in focus over awide range of brightnes adjustment. For a more complete understanding ofthe invention, reference should now be had to the drawing, in which thesingle figure constitutes a diagrammatic representation of a portion ofa television'receiver, including a picture or cathode ray tube providedwith brightness and focus control means embodying the present invention.

Referring now to the drawing, the invention is in a television receiverof the superheterodyne type, including an antenna and ground system Inand II connected to a radio frequency amplifier l2, to which areconnected in cascade, as shown, and in the order named, an oscillatormodulator IS, an intermediate frequency amplifier II, a detector and AVCsupply IS, a video frequency amplifier IE, and a cathode ray, signalreproducing tube, or picture tube I7, which elements constitute the mainor video signal channel of the receiver. The unidirectional bias voltagedeveloped by the AVG supply [5 is applied by suitable leads l8 to one ormore of the control grids of the tubes contained in the preceding stagesof the signal channel in a conventional manner, and since the elementsor components II to Hi, inclusive, are all of conventional well knownconstruction, no detailed illustrations and descriptions thereof aredeemed .necessary.

As shown, the picture tube l1 includes the usual focusing coil IS, amain or video'control grid 20,

and a cathode 2|, the control grid 2| bein connected through suitablecoupling means, such as the condenser 22, to the output of the video amplifier Hi. It will be understood, of course, that the tube I1 likewiseincludes the usual additional elements such as first and second anodes,a fluorescent screen at the end of the tube opposite the cathode 2|, andsuitable deflecting or scanning means, but sinc these additionalelements are well known in the art and constitute no part of the presentinvention, it is not deemed necessary to illustrate or describe eitherthese elements or thevar'ious control circuits therefor, which maylikewise be entirely conventional.

As shown in the drawing, the focusing coil I9 of the picture tube I1 isconnected in a focusing control circuit which extends from a suitablesource of potential, represented by the symbol B+, through a conductor23, the focusing coil I9, a conductor 24, a focus controllingpotentiometer 25, consisting of the usual resistance element 26 andadjustable contact 21, a conductor 28 and-the constant resistanceportion 29 of a .brightness controlling potentiometer 30, and by way ofa fixed resistor 3| to a suitable ground connection 32; The brightnesscontrolling potentiometer is provided with the usual adjustable 3contact for arm 33, which is connected through a suitable conductor 34to the cathode 2| oi the tube l1,.so'as :to apply an adjustablefbiasvoltage thereto, "and in accordance with the present invention asuitable resistor or impedance 35 is connected between the adjustablearm 33 of the potentiometer 30 and the lower terminal 36 .of theresistance element 29, so as to be efiectively connected in parallelwith the adjustable portion of the potentiometer 30 which*is'iin'clude'd inboth the brightness control and'iocus controlcir- Icuits as heretofore described.

It will be apparent that, day adjustmentsof ,the movable arm 21 of thepotentiometer 25, the current flow through the focusing coil I 9 may beadjusted so as to bring the tube l1 into focus, rand kit.tvill'ilikewise be apparentrthat adjustment of the movable arm 33 ofthe potentiometer ifll *ls effective :to adjust 'the :bias *on the(cathode 2 l pf thertu'be IrksocaswtorproVide arange of bright- What isclaimed and desired to be secured by Letters Patent is: 1..Inaicorrtrolrapparatusiior"atmagnetic-focus cathode ray tube havingindependently adjustable potentiometer means for respectively con-"'--at'ro1ling the brightness and the focus of said stube saidpotentiometer means being connected '-ness-"-w-liich varies between adesired low -intensity and high :intensity picture. Since adjust- "mentof 'zthe .imovable :arm .33 does "not wof itself changeithe-efiect of:the resistance 29111180 'fana's "the focusingvcontrol circuit isconcerned, changing the brightness :adjustmentwould result in defocusingof t-hetube were it not for =:the automaticscoritrol.efifecitedby theresistor or impedancef35. "This impedance "35, "however, which ismonnected in :the docusing-control circuit in par-:zfllele'circuitrelation,as heretofore described, with"ewhich'aoccurswhen the "impedance 35 has :been

icompletely shorted out, leaving only theresistor 331 ,"thepotentiometer :resistor 29, and a portion 10fthe-potentiometer:resistor:26 in .the focus conitrol circuit.

In one application of the "-invention,1wherein the tube .111 took theform of a conventional 'tw'elve-inch picture tube of the magnetic focustype, the potentiometer resistors 26'and 29 were "respectively givenvalues :of 2-5 ,000 tohms and 2 ,000

fihms, the impedance '35 .took the form :of 2a 15000 ohm ir'esistor, theresistor 31 'had a value of 3820 -'=o'hms, sand the" source of energy-'B+ constituted a' 400'volt source, it was found thatthe tubell, setterh-aving been properlyiocuse'd "by adjustment oi' the arm 21, remained in*focus during adjustifinentof the brightness between abiasing'potentia'l-of 38 volts and 18 volts; the brightness varia-'=ition thereby ieffiecte'd :ranging from a picture of ilow v brightness.to one of'very high intensity. "When-conventional*prior artfocusing'and'brightness controlling-arrangements were employed 1 inconnection with the :same tube, it was not iios- "-'sible to maintainthe tube in focus over a range {"56 morethan six volts biasingpotential, which -eonsti'tutes approximately one-third of the'range@provided by control means embodyingthis invention.

While a particular embodiment of theinvendzion'has been shown, it willbeunderstoodpo'f -course, that the invention is not limited thereto 7 1 in-,series-.circuit relation to form a focus control circuit comprising aconstant portion of said brightness :controlling potentiometer means andan adjustableportion of said focus potentiometer means and a'brig-htness control circuit compristingv -the" combination of impedancemeans connested in parallel circuit relation with an ad-'jus'tableportion of said brightness controlling potentiometer means formaintaining said tube in focus "DWI *BnWide range of brightness adjust-"2. Gont-rol :apparatus *for a magnetie-imeus *cathode ray tubecomprising, adjustable impedance imeans for :controlling the currentiflow lthroughgtheiocus coil-of said tube, independently adjustablebrightness controlling -impedanee means for controlling the bias voltageappliedti'n said tube, and additional impedance =means adjustableebysaid brightnesscontrolling -meanseior controllingvthe current :flowthrough said'scoil-to maintain said tube in focus overa wide-tangentbrightness adjustment.

'3. 'Control --apparatus for :a magnetic-focus cathode'ray tube lhavingia focus coil, comprising a pair of adjustable :potentiometers, meansconnecting an 1 adjustable portion-of one ief 'sai'd potentiometers and@a constant portion oi the other of said potentiometers in circuit withcoil to :provide a focus -contro1ling'circuit wherein thecurrentfiowthroughrsaidcoil is-adjustable by said :one-potentiometer,--means connecting sen adjustable portion of saidotherflpotentiometerdzo said tube :for applying I a brightness-controllingvariable bias voltage thereto, :and impedance -means-in-saidfocus'zcentrolling Ecircuit effectively adjustable by adjustment 01 saidother P11012811- -tiometer for varying :said current -fl'ow to mien tainsaid tube in -focus over a wide range sof --brightness-adjustment.

'4. .Contro1 :apparatus for a rmag-neticwfoous tcathode-ray=tube=ha-vinga'focus .coil, comprising a :pair of zadjustable potentiometers,means :connecting :anadjustable portion-of one of :saidpotentiometersand a constant ;portion-iof the rOthflT of ,said potentiometers-incircuit -with-said-=coil to provide a .focuscontrolling :circuit whereinthe current flow through 'said coil is adjustable by saidionepotentiometer, means connecting erredjustable ,portionof said otherpotentiometer -;t0 said tube -.for applying .a brightness controllingvariable. bias voltage thereto, and impedance means connectedin parallelcircuit relation with said adjustableportionbf said other ,potentiometerformaintaining said tube in focus .upon adjustment'of said otherpotentiometer to varysa'id brightness controlling "bias voltage.

RALPH '0. HOLBROOK.

REFERENCES CITED UNITED STATES wPATEN'I'S

